This invention relates to bone fracture fixation devices, and more particularly to systems that involve positioning of an intramedullary nail within the intramedullary canal, followed by cross-locking for fixation of the intramedullary nail to achieve bone fixation.
Intramedullary fixation is a well-accepted technique for internal fracture fixation of long bones, typically the femur or the tibia, although humeral and forearm (radial or ulnar) applications also are possible. This fixation technique involves inserting an intramedullary nail, usually a hollow shaft having a slight bend or curvature, into the intramedullary or marrow canal. Once inserted and properly positioned within the bone, the intramedullary nail is fixed to the bone by cross-locking, with screws extended transversely with respect to the elongated nail through the bone, and through holes in the intramedullary nail, or in the case of hollow nails, through diametrically opposed holes in the nail wall.
The cross-locking fixation technique is shown in U.S. Pat. No. 5,122,141 (Simpson, et al.), which also illustrates an inclined disposition of bone screws through the nail at the proximal end of the femur. U.S. Pat. No. 5,112,333 (Fixel) also illustrates an intramedullary nail secured in the femur using fasteners directed transversely of the nail.
FIG. 1 illustrates an intramedullary nail 1 within the intramedullary canal 2 of a long bone 3, for example the femur. The nail is fixed by two bone screws 4 and 5. Bone screw 4, extended through a wall 6 of the bone on opposite sides of the intramedullary nail, also extends through diametrically opposed holes 7 and 8 through the nail wall to secure the intramedullary nail within the intramedullary canal. Bone screw 5 extends in similar fashion through the bone wall and through openings 9 and 10 through the nail wall, to further secure the nail.
FIG. 2 is an enlarged view showing a portion of bone screw 4 extending through hole 7. The bone screw has an elongated shank 11 having a shank diameter constituting a xe2x80x9cminorxe2x80x9d diameter of the screw. An external thread 12 surrounds the shank, with the radial extremity of the thread determining a major diameter of the screw. The diameter of hole 7 closely approximates the major diameter of the bone screw, so that the thread extremity establishes a substantially helical contact or interface with the intramedullary nail along the wall defining hole 7.
Although this arrangement has in general been satisfactory, several difficulties arise due to the amplitude and direction of stresses at the intramedullary nail/screw interface. More particularly, both the tibia and the femur are required to support substantial body weight, and thus are subject to substantial axially directed compressive forces and substantial shock in the axial direction. The muscles also can exert twisting forces upon the bone. An intramedullary fixation system is subject to these same forces.
Fasteners such as bone screw 4 are designed primarily to bear loads in the axial direction with respect to the fastener, and thus are well suited for certain uses, e.g. securing bone plates. However, when used to interlock an intramedullary nail, the bone screw is subject to the aforementioned axial compressive stresses and twisting, which operate as sheer forces directed laterally or transversely with respect to the screw. In some cases, the sheer forces are of sufficient magnitude to fracture or break the bone screw at a point near the intramedullary nail hole that accommodates the screw.
One attempt to solve this problem involves using larger-diameter bone screws. A consequence of using larger screws is that the holes through the intramedullary nail needed to accommodate the screws must also be larger, which compromises the integrity of the nail. Accordingly, although larger screws may reduce the risk of screw failure due to sheer, they are likely to increase the risk of nail failure.
Another approach is to form the bone screws from a material selected for a high resistance to fracture, for example stainless steel. The materials selected to form the intramedullary nail and bone screws, however, must have a high degree of biocompatibility as well. Titanium and certain titanium-based alloys are highly preferred for their biocompatibility, despite their notch sensitivity characteristics as compared to stainless steel. Steel components lack the degree of biocompatibility desired in many applications. A xe2x80x9cpartial solutionxe2x80x9d of using a titanium intramedullary nail in combination with steel bone screws would not be satisfactory, due to galvanic corrosion at the nail/screw junctions.
Another approach addressing this problem is seen in U.S. Pat. No. 5,814,047 (Emilio, et al.). The Emilio patent describes a fixation system in which the intramedullary nail is secured by several flexible screws with distal end portions slightly inclined relative to the longitudinal nail extension, as opposed to more rigid, transverse screws. This arrangement, however, requires elongate flexible screws of different lengths, and structure within the nail for channeling these screws and diverting the tips at a slant relative to the nail.
Another problem caused by stresses laterally of the bone screws is a risk of plastic deformation of the screw threads, the interior of the holes through the nail wall accommodating the screws, or both as a result of the forces involved. For example with reference to FIGS. 2 and 3, as threads 12 and the internal surface of hole 7 are urged against one another, there is a high stress concentration along the thread/wall interface which can tend to flatten the external threads, or lead to depressions in the hole wall, or both, as indicated by the broken lines in FIG. 3. In any of these events the integrity of fixation is compromised. Any transverse loads can cause further plastic deformation, and may further compromise fixation.
Therefore, it is an object of the present invention to provide an interlock screw for securing an intramedullary nail, with an improved capacity to withstand forces directed laterally with respect to the screw, i.e. in directions perpendicular to the screw length.
Another object is to provide an intramedullary interlock screw with an external thread providing a larger area of contiguous surface contact at the interface with an intramedullary nail secured by the screw in a bone fixation application.
A further object is to provide a bone fixation system in which the components can be formed from a wider variety of materials, and yet maintain desired levels of strength and resistance to fatigue.
Yet another object is to provide an intramedullary interlock screw that has a reduced major diameter such that openings in intramedullary nails to accommodate the screws can be reduced in size, while maintaining in the screw a desired resistance to bending under laterally applied forces.
To achieve these and other objects, there is provided a fastener for securing a fixation member with respect to osseous material. The fastener includes an elongate shank formed of a biocompatible material and extended in an axial direction. The shank has a maximum shank diameter and a shank outer surface. An external thread, formed of a biocompatible material, is disposed helically about the shank. The external thread has a substantially uniform thread pitch in the axial direction and defines a thread outer surface substantially parallel to the shank outer surface and spaced apart from the shank outer surface by a thread height. The width of the thread outer surface in the axial direction is at least twenty percent of the thread pitch, and the maximum shank diameter is at least six times the thread height.
As compared to the previously known bone screw shown in FIGS. 1-3, the width of the thread outer surface, i.e. the crest length, is considerably larger in proportion to the pitch length. Also as compared to the known screw, the maximum shank diameter is at least six times the thread height, thus to provide a thread height or depth considerably less in proportion to the screw size than the height in the previously known bone screw.
Several advantages arise from the foregoing features. First, increasing the ratio of the shank diameter with respect to the radial depth or height of the threads, increases the shank size in proportion to the size of the screw. For a screw with a given major diameter, this increases the strength of the screw, particularly in terms of its capacity to resist bending in response to sheer forces, i.e. the forces typically directed longitudinally of the intramedullary nail and transversely of the interlock screw. In particular, because the resistance of the shank to bending increases in proportion to its diameter to the fourth power, a slight increase in shank diameter results in a considerable increase in strength.
The increase in crest length with respect to pitch increases the area of contiguous surface contact between the most radially outward surface of the threads, i.e. the crest, and the wall portion of the intramedullary nail forming the opening in which the screw resides. The compressive forces that drive the intramedullary nail against the screw are distributed over a larger surface area, reducing stress concentrations sufficiently to virtually eliminate plastic deformation of the threads or wall of the nail surrounding the threads. This maintains the integrity of the screw/nail coupling, for a more secure fixation of the intramedullary nail.
The increase in crest length in proportion to pitch also increases the strength of the interlock screw, because it increases the proportion of the overall screw length having the major (crest) diameter and diminishes the proportion having the minor (shank) diameter.
Another aspect of the present invention is an intramedullary interlock screw. The screw includes an elongate shank extended in an axial direction, and having a cylindrical shank outer surface defining a shank diameter. An external thread is disposed helically about the shank and has a thread outer surface substantially parallel to the shank outer surface. The thread outer surface is coaxial with the shank outer surface and defines a thread diameter. The external thread further has a thread pitch and a thread width in the axial direction. The thread width is at least twenty percent of the thread pitch.
More preferably, the thread width is about one-half of the thread pitch. Thus, the proportion of the screw length having the major (thread) diameter is increased and the proportion of the length having the minor (shank) diameter is reduced, enhancing the strength of the screw in terms of resisting bending in response to laterally applied forces.
According to another aspect of the present invention there is provided an interlock screw adapted to withstand lateral forces. The interlock screw includes an elongate shank having a shank axis and a cylindrical shank outer surface defining a shank diameter. An external thread is disposed helically about the shank and has a substantially uniform thread pitch in the axial direction. The thread is concentric on the shank and has a thread outer surface substantially parallel to the shank outer surface and spaced apart from the shank outer surface to define a thread diameter. The shank diameter is at least about 0.75 times the thread diameter.
Preferably the shank diameter is in the range of 0.8-0.9 times the thread diameter. More preferably, the shank diameter is at least 0.85 times the thread diameter.
Advantageously the external thread further has a thread width, in the axial direction, at least about 0.2 times the thread pitch.
The external thread can be formed with opposite side walls disposed between the shank outer surface and the thread outer surface. Preferably the side walls are inclined with respect to planes perpendicular to the shank axis. Further, junctions of the side walls with the shank outer surface are preferably rounded as opposed to forming sharp corners or edges. Junctions of the side walls with the thread outer surface likewise are preferably rounded. This tends to reduce stress concentrations, and is particularly advantageous when titanium or titanium-based alloys are used to form the interlock screw.
Yet another feature of the invention is a bone fixation system including an elongate intramedullary nail, at least one opening formed through the intramedullary nail at a first end region of the nail, at least one opening formed through the intramedullary nail at a second end region opposite the first end region, and a plurality of the interlock screws constructed according to the present invention, one such screw associated with each of the openings.
Thus in accordance with the present invention, intramedullary locking screws can be formed of materials selected for a high degree of biocompatibility, and with a desired level of resistance to laterally applied bending forces, without unduly enlarging the major (thread) diameters. The interlock screw further reduces the risk of plastic deformation of the screw threads and the portion of the intramedullary nail surrounding and contacting the screw threads. As a result, the screw-accommodating holes in intramedullary nails can be kept smaller, to better ensure the structural integrity of the nails.